Water or waste water treatment methods often include the step of directing the water through a filter. The filter traps most of the solids that are suspended in the water. The filter is periodically cleaned to remove the trapped solids.
One type of filter is known as a floating bed or buoyant media filter. Such a filter comprises buoyant particles that are contained within a vessel. The unfiltered water is directed to flow upwardly through the particles.
Buoyant media filters can be easily cleaned. In this regard, the buoyant media may be dispersed and agitated by a backflow of water into the vessel or by the introduction of gas bubbles into the liquid-filled vessel. Dispersion and agitation of the filter media releases the trapped solids. The released solids settle to the bottom of the vessel and are removed from the vessel through a drain. After completion of the filter-cleansing mode of operation, the buoyant media floats back into position at the surface of the liquid in the vessel.
The effectiveness of a filter is primarily a function of the length of the flow path of the water through the filter. More particularly, the filter performance (measured, for example, as the percentage of solids removed from the water) increases with the length of the flow path through the filter media. Accordingly, the filter performance of buoyant media filters is increased by increasing the depth of the media through which the water vertically flows.
Increasing the flow path of any vertical-flow filter substantially increases the cost and complexity of the filtration system because of the need for constructing a vessel that is suitable for supporting a deep column of liquid and filter media.